Electrodes for Batteries and Methods for Making Same

ABSTRACT

A method of fabricating a battery electrode includes forming a mixture including an electrode material and a binder; forming an electrode blank from the mixture; heating the electrode blank at a predetermined temperature for a predetermined time to form an annealed electrode blank; and laminating the annealed electrode blank to a current collector. The current collector may include a conductive carbon coating. In such event, the method may further include heating the current collector at a selected temperature for a selected time prior to laminating the annealed electrode blank to the current collector.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of the filing date of U.S.Provisional Application No. 62/627,060, filed Feb. 6, 2018, thedisclosure of which is hereby incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

The present disclosure relates generally to batteries and to methods formaking same. More particularly, the present disclosure relates tomethods for making improved battery electrodes and to batteriesincorporating same.

BACKGROUND OF THE INVENTION

Batteries used in Implantable Medical Devices (IMDs), such as cardiacpacemakers and implantable cardioverter defibrillators, are required tomeet high quality and performance specifications and reliability. Sincethe replacement of the battery in an IMD requires the patient to undergosurgery, batteries for IMDs must have a long service life. Improvementsto the reliability, performance, and lifetime of such batteries istherefore highly desirable.

Apart from battery chemistry, the process by which battery electrodesare manufactured is one of the most important factors impacting thequality and performance of the battery. Different fabrication methodshave been used to produce electrodes used in battery production. Onecommon method is slurry coating or tape casting, in which a pasteincluding an electrode material and a binder is coated onto a metal foilcurrent collector using a slot die or a doctor blade. Such processes areordinarily used for battery applications requiring thin electrodes, suchas high rate lithium ion batteries. In another method of producingelectrodes, the electrode material is mixed with a binder, formed into asheet, cut to size, and then pressed onto a metal mesh currentcollector. The binder acts to hold the electrode material together andto help attach it to the current collector. However, the high pressurepressing step creates high stresses in the binder that commonly causewarping of the electrode and delamination of the electrode material fromthe current collector. Furthermore, the electrode material generallydoes not bind well to the current collector and needs to permeatethrough the mesh holes of the current collector to bind to the electrodeactive material on the other side of the current collector. The failureof this binding contributes to the delamination problem.

Another issue in electrode production lies in the current collectoritself. The current collector provides mechanical support to theelectrode materials and conducts current from the electrodes to thebattery terminals. As the current collector is a non-active component ofthe battery, it is desirable to reduce the volume of the currentcollector as much as possible in order to maximize the energy density ofthe battery. Where the electrode material is not compatible with thematerial forming the current collector, a conductive coating is oftenapplied to the surface of the current collector to protect it fromreacting with the electrode material. Defects in this coating could havea detrimental impact on the long-term performance of the battery.

There therefore is a need for improvements to the methods used toproduce battery electrodes in order to address these problems.

SUMMARY OF THE INVENTION

One aspect of the present disclosure provides a method for fabricating abattery electrode. The method includes forming a mixture including anelectrode material and a binder; forming an electrode blank from themixture; heating the electrode blank at a predetermined temperature fora predetermined time to form an annealed electrode blank; and laminatingthe annealed electrode blank to a current collector.

Another aspect of the present disclosure provides a battery including ahousing; and a cell stack disposed in the housing, the cell stackincluding an anode, a cathode made by the method described above, and aseparator electrically insulating the anode from the cathode.

Yet a further aspect of the present disclosure provides another methodfor fabricating a battery electrode. The method includes forming amixture including an electrode material and a binder; forming anelectrode blank from the mixture; heating a current collector having aconductive coating at a selected temperature for a selected time to forman annealed current collector; and laminating the annealed currentcollector to the electrode blank.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present battery electrodes and methods formanufacturing same are disclosed herein with reference to the drawings,wherein:

FIG. 1 is a schematic cross-section of a battery according to oneembodiment of the disclosure;

FIG. 2 is an exploded perspective view of a battery cathode assembly;

FIG. 3A is a highly schematic side view of a cold pressed cathodeassembly;

FIG. 3B is a highly schematic side view of a cathode assembly that wasannealed prior to pressing;

FIG. 4 is a graph illustrating battery acceptance pulse voltages forbatteries having cathodes formed by different processes;

FIG. 5 is a graph illustrating accelerated pulse test results forbatteries having cathodes formed by different processes; and

FIG. 6 is a graph illustrating the depth of discharge for batterieshaving cathodes formed by different processes.

DETAILED DESCRIPTION

The present application describes methods for producing electrodeassemblies, and in particular cathode assemblies for use in medium ratebatteries. Such batteries typically have a single relatively thickcathode assembly and one or two anode assemblies separated from thecathode assembly by a separator. However, the methods disclosed hereinare not limited to medium rate batteries, but may be applied to anybatteries having one or more electrodes formed using a pressingoperation, including high rate batteries, rechargeable batteries andother types of batteries.

As used herein, the terms “substantially,” “generally,” and “about” areintended to mean that slight deviations from absolute are includedwithin the scope of the term so modified.

General Battery Design

The battery embodiments described herein may be particularly useful tothe operation of IMDs, such as those that employ medium rate batteries.Examples of such devices include pacemakers and implantable cardiacmonitors.

IMDs require a power source in order to operate. A primary lithiumbattery may be used to provide a medium current output power source.FIG. 1 illustrates an exemplary design for a medium rate battery 100.Battery 100 includes a cathode or cathode assembly 102, and a pair ofanodes or anode assemblies 104 sandwiching the cathode (although it willbe appreciated that battery 200 may include a single anode 104). Eachanode 104 is separated from cathode 102 by a separator 106, such asmicroporous polypropylene or polyethylene (or a combination of both).

Cathode 102 and anodes 104 may each include some active material bondedto a current collector. The active materials take part in theelectrochemical reaction to produce the current, while the currentcollectors are conductive materials that provide a low-resistance pathfor the current to flow. For example, each anode 104 may include ananode active material 110 bonded to a current collector 112, whilecathode 102 may include a cathode active material 114 mixed with otheradditives and bonded to opposite sides of a current collector 116. Theassembly of anodes 104, cathode 102 and separators 106 may be arrangedin an insulating housing 120. The current collectors 112 of anodes 104may be electrically connected together and joined to the negativeterminal 122 of battery 100, while the current collector 116 of cathode102 may be joined to the positive terminal 124 of battery 100.

Although not shown in FIG. 1, housing 120 is filled with an electrolyteto facilitate ion transport between anodes 104 and cathode 102. Theelectrolyte may be a polymer or a liquid electrolyte as would beunderstood by one skilled in the art. Examples of the electrolytesystems include lithium bis-trifluoromethanesulfonimide (LiTFSI) inpropylene carbonate/dimethoxyethane, lithium hexafluoroarsenate (LiAsF₆)in propylene carbonate/dimethoxyethane, lithium hexafluorophosphate(LiPF₆) in propylene carbonate/dimethoxyethane, lithiumbis(fluorosulfonyl)imide (LiFSI) in propylene carbonate/dimethoxyethane,lithium tetrafluoroborate (LiBF₄) in gamma-butyrolactone, or lithiumtetrafluoroborate (LiBF₄) in gamma-butyrolactone/dimethoxyethane. Othersuitable electrolyte systems may be used.

In some embodiments, the solvents used in the electrolyte may beselected from the group consisting of propylene carbonate (PC),dimethoxyethane (DME), ethylmethyl carbonate (EMC), dimethyl carbonate(DMC), diethyl carbonate (DEC), or gamma-butyrolactone (GBL). Othersuitable solvents may be used in combination with the electrolyte salts.

In some embodiments, some additives may be added to the electrolyte incombination with the solvents. The additives may be selected from thegroup consisting of diphenol carbonate (DPC) or dibutyl carbonate (DBC).Other suitable additives may be used.

In one embodiment, the active material 110 of anode 104 may comprise alithium foil, which may be bonded to current collector 112 consisting ofan unperforated foil of metallic nickel. Cathode 102 may consist of acathode active material 114 bonded to a perforated titanium mesh currentcollector 116. In some embodiments, the titanium mesh may be coated witha layer of a conductive material to prevent reactions between thetitanium mesh and the cathode active material. Particularly usefulmaterials for this purpose include conductive carbon paints, such as DAGEB-012 and DAG EB-815 carbon paints available from Henkel AG & Co. ofDusseldorf, Germany.

Cathode active material 114 may include silver vanadium oxide (SVO),sub-fluorinated carbon fluoride (CF_(x)), a combination of SVO andCF_(x), or other known cathode active materials. Where SVO and CF_(x)are used in combination, cathode 102 may include individual layers ofCF_(x) and SVO bonded together, or the CF_(x) and SVO may be mixedtogether to form a single homogenous layer. Typically, for CF_(x), x maybe between about 0.6 and about 1.2, and in a particular example may beabout 1.1. The CF_(x) may first be treated with a base, such as ammoniumhydroxide, in order to neutralize acidics in the CF_(x) which candegrade the electrolyte, leading to an overall increase in theresistance of the battery. After treatment, the CF_(x) material may berinsed with a solvent, such as ethanol.

Cathode Fabrication

In one embodiment, about 15 wt % SVO is blended with about 85 wt %CF_(x) to form a mixture. To increase the conductivity of the mixture,an additive may be added. The additive may include, for example, one ormore of carbon nanotubes, carbon black, graphene, or metalnanoparticles. For example, between about 1 wt % and about 20 wt % ofcarbon black with a surface area of around 60 m²/g may be added to themixture to improve the conductivity of the cathode active material.

In addition to a conductivity aid, the mixture may include a binder tohold the materials together following a pressing step. Suitable bindersinclude polytetrafluoroethylene (PTFE), polyvinylidene difluoride(PVDF), and others. In one embodiment, between about 1 wt % and about 10wt % of PTFE binder may be added to the mixture.

In a particular embodiment, the SVO, CF_(x), PTFE binder and carbonblack additive may be blended together in an isoparaffin medium to forma slurry. The slurry may include between about 20 wt % and about 60 wt %of the dry ingredients and between about 40 wt % and about 80 wt % ofthe isoparaffin. The isoparaffin may be IsoPar-G, which is a mineralspirit. The slurry may be dried using a centrifuge and cast into thicksheets. The sheets may then be dried under vacuum and cut into cathodematerial blanks 114 of a desired size and shape to form cathode 102. Themethod for forming cathode blanks 114 may include additional and/ordifferent steps, or the blanks may be made by a different method as willbe known to those skilled in the art.

In a typical cathode lamination process, referred to herein as a coldpressed process, a cathode collector 116 is stacked between two cathodeblanks 114 and compressed at room temperature by a hydraulic die pressto laminate the layers together to form cathode assembly 102. Thelamination of cathode blanks 114 with current collector 116 inducessignificant stresses within the PTFE binder. In the cold pressedprocess, these stresses often cause warping of cathode assembly 102 anddelamination of cathode material 114 from current collector 116. FIG. 3Ais a highly schematic side view showing the warping and delamination ina cathode assembly 102 formed by a cold pressed process.

Several attempts were made to alter the lamination process to avoid thedefects resulting from the cold pressed process. In one attempt,referred to herein as the hot pressed process, the die of the hydraulicpress was heated to 220° C. such that cathode blanks 114 were heatedduring the pressing step. Although the resulting cathode assembly 102was flat and did not exhibit delamination, cathode material 114 stuck tothe pressing die and the cathode was hard to demold, especially whilethe die was hot. In a third lamination process, referred to herein ascold pressed and post-annealed, cathode blanks 114 were laminated tocurrent collector 116 at room temperature as in the cold pressedprocess, and the resultant cathode assembly was subsequently clampedunder pressure and annealed at 280° C. for 30 minutes. This processproduced cathode assemblies 102 that are flat and that do not exhibitdelamination, but the electrodes stuck to the die during the annealingprocess.

In a fourth process, referred to herein as the pre-annealed and coldpressed process, cathode blanks 114 were annealed prior to a coldpressing step. In an annealing process, cathode blanks 114 are heated toa predetermined temperature for an appropriate amount of time. The timeand temperature of the annealing process will be influenced by severalfactors, including the thickness of the cathode blank, and the type andamount of binder used. For example, for a PTFE or PVDF binder (or acombination of the two), the annealing process may be conducted at atemperature of between about 100° C. and about 320° C. for between about5 minutes and about 6 hours. Preferably, the annealing process for thesebinders may be conducted at a temperature of between about 200° C. andabout 300° C. for between about 10 minutes and about 1 hour. In aspecific embodiment in which the cathode blank material includes about 2wt % PTFE binder, the annealing process may be conducted at about 280°C. for about 30 minutes. The annealing process is thought to greatlyreduce the stresses that develop in the PTFE binder during thesubsequent lamination process.

Following the annealing process, cathode blanks 114 may be cooled toroom temperature. A cathode collector 116 may then be stacked betweentwo cathode blanks 114, as shown in FIG. 2, and inserted into ahydraulic die press to laminate the layers together into cathodeassembly 102. Cathode blanks 114 and current collector 116 may belaminated together at room temperature and a pressure of between about 5ksi and about 100 ksi. The pressure in ksi refers to kilopounds persquare inch. In one example, a preferred pressure range is about 30 ksito about 60 ksi, and more preferably about 40 ksi, with an activeloading range of between about 71 mg·cm⁻² and about 73 mg·cm⁻². As shownin FIG. 3B, cathode assembly 102 resulting from the pre-annealed andcold pressed process is flat and does not exhibit delamination betweencathode material 114 and current collector 116.

Following lamination, cathode assembly 102 may be encapsulated byseparators 106, shown in FIG. 2. Each cathode assembly 102 may be sealedin a shut-down separator bag or sleeve made of an electricallyinsulating material to electrically insulate cathode assembly 102 fromanodes 104 and housing 120, while still allowing the transport of ions,particularly Li+ ions, therethrough to facilitate the passage ofelectric current in battery 100. In some embodiments, separators 106 maycomprise a micro-porous or nano-porous material with an average poresize between about 0.02 μm and about 0.5 μm. In an exemplary embodiment,the average pore size in separators 106 is about 0.05 μm. Separators 106(or the shut-down separator bag) may be made, for example, from amaterial selected from the group consisting of paper, cotton, nylon,polyethylene, polypropylene, polytetrafluoroethylene, ceramics orrubber. Other suitable materials may be used.

Further improvements to cathode 102 may be achieved by processingcurrent collector 116 prior to the lamination step described above.Current collectors 116 are frequently supplied by an outside vendor thatmanufactures the current collectors and/or coated current collectorsusing proprietary technology. It has been found that the conductivecoating applied to current collectors 116 exhibits microscopic cracksand pin holes. Additionally, the coatings in these current collectorsmay not be fully cured as received from the supplier, and may includesolvent remnants distributed therein. The defects in the coating and anysolvent remnants may have a detrimental impact on the long-termperformance of battery 100. Since the conductive coatings typicallyapplied to the current collectors include a wax or similar binder tohold the coating together and adhere it to the underlying titanium mesh,subjecting the current collectors to an annealing process prior to theirlamination to cathode blanks 114 may drive off any remnant solvents andreflow the coating to fill any cracks and pin holes therein, and thusmay improve the long-term performance of battery 100.

The process for annealing current collector 116 is similar to that forannealing cathode blanks 114. That is, in the annealing process, currentcollector 116 is heated to a selected temperature for an appropriateamount of time. The time and temperature of the annealing process willbe influenced by factors such as the amount and type of the wax or otherbinder in the coating, the coating thickness, and the like. For currentcollectors 116 having a DAG EB-012 or a DAG EB-815 carbon coating, theannealing process may be conducted at a temperature of between about120° C. and about 300° C. for between about 5 minutes and about 120minutes. In a specific embodiment, the annealing process may beconducted at about 280° C. for about 30 minutes. Following annealing,current collector 116 may be laminated between two cathode blanks 114using the pre-annealed and cold pressed process described above.

Battery 100 may be formed by stacking cathode assembly 102 encapsulatedby separators 106 between two anodes 104. Anodes 104 may also beencapsulated by separators (not shown) that may include a shut-downseparator bag or sleeve of the type described above. The stackedarrangement may then be assembled within housing 120, and the housingmay be filled with an electrolyte, such as lithium tetrafluoroborate(LiBF₄) in gamma butyrolactone/dimethoxyethane. The cathode assembly 102in the stack may include a coated current collector 116 annealed by theprocess described above, and two cathode blanks 114 laminated to thecurrent collector using the pre-annealed and cold pressed processdescribed above.

Batteries produced by the methods described herein were tested forelectrical performance FIG. 4 shows the acceptance pulse (AP) voltagefor batteries in which cathode assembly 102 was formed using thedifferent lamination processes described above. In each case, thecurrent collector 116 was as received from the vendor (i.e., notannealed). In FIG. 4, graph a1 reflects the AP voltage for a batteryhaving a cold pressed cathode assembly; graph a2 reflects the AP voltagefor a battery having a cold pressed and post-annealed cathode assembly;and graph a3 reflects the AP voltage for a battery having a pre-annealedand cold pressed cathode assembly. As can be seen, the battery havingthe pre-annealed and cold pressed cathode assembly yielded the highestAP voltage, which corresponds to the best battery performance.

FIG. 5 illustrates the accelerated pulse test (APT) voltage forbatteries in which cathode assembly 102 was formed using the differentlamination processes described above. In each case, the currentcollector 116 was as received from the vendor (not annealed). In FIG. 5,the x axis depicts the depth of discharge (DOD), which is the percentageof the battery capacity discharged. There are two groups of lines foreach battery tested. The dashed lines in the graph depict the voltagesat the end of a 20 mA pulse, while the solid lines in the graph depictthe background voltage at 0.55 mA load. Graphs b1 and b4 reflect the APTvoltages for a battery having a cold pressed cathode assembly; graphs b2and b5 reflect the APT voltages for a battery having a cold pressed andpost-annealed cathode assembly; and graphs b3 and b5 reflect the APTvoltages for a battery having a pre-annealed and cold pressed cathodeassembly. As reflected in the graphs, the battery having thepre-annealed and cold pressed cathode assembly yielded the highest APTvoltage at each percentage of discharge, reflecting the best batteryperformance.

Electrical tests were also performed to determine the performance of avariety of different current collectors, including coated currentcollectors that were subjected to an annealing process as describedabove. Batteries having different cathode current collectors weresubjected to a 3-month 72° C. accelerated depth of discharge (ADD) lifetest. The batteries were pulsed using constant current for 15 minutesand the battery voltage was recorded during the pulsing. FIG. 6 showsthe voltage at the end of the pulsing, with the x axis depicting thedepth of discharge. Graph c1 reflects the voltage for a battery havingan uncoated titanium current collector; graph c2 reflects the voltagefor a battery having a DAG EB-012 coated titanium current collector asreceived from the vendor (not annealed); graph c3 reflects the voltagefor a battery having a DAG EB-815 coated titanium current collector asreceived from the vendor (not annealed); graph c4 reflects the voltagefor a battery having a DAG EB-012 coated titanium current collectorannealed at 280° C. for 30 minutes; and graph c5 reflects the voltagefor a battery having a DAG EB-815 coated titanium current collectorannealed at 280° C. for 30 minutes. As can be seen from these graphs,the battery with the uncoated titanium current collector died quickly.Both the DAG EB-012 and DAG EB-815 coated current collectors performedbetter, while the best performance was achieved by the batteries inwhich the coated current collectors were subjected to an annealingprocess.

To summarize the foregoing, according to a first aspect of thedisclosure, a method for fabricating a battery electrode includesforming a mixture including an electrode material and a binder; formingan electrode blank from the mixture; heating the electrode blank at apredetermined temperature for a predetermined time to form an annealedelectrode blank; and laminating the annealed electrode blank to acurrent collector; and/or

the electrode material may include a cathode active material; and/or

the cathode active material may include sub-fluorinated carbon fluoride(CF_(x)); and/or

the cathode active material may include silver vanadium oxide (SVO);and/or

the mixture may include a conductivity enhancer selected from the groupconsisting of carbon nanotubes, graphene, carbon black, metalnanoparticles, and combinations thereof; and/or

the binder may be selected from the group consisting of polyvinylidene,polytetrafluoroethylene, and combinations thereof; and/or

the laminating step may include bonding a first electrode blank to afirst side of the current collector and bonding a second electrode blankto a second side of the current collector; and/or

the method may further include the step of cooling the annealedelectrode blank to about room temperature prior to the laminating step;and/or

the predetermined temperature may be between about 100° C. and about320° C.; and/or

the predetermined temperature may be between about 200° C. and about300° C.; and/or

the predetermined temperature may be about 280° C.; and/or

the predetermined time may be between about 5 minutes and about 6 hours;and/or

the predetermined time may be between about 10 minutes and about 1 hour;and/or

the predetermined time may be about 30 minutes; and/or

the current collector may include a titanium mesh; and/or

the current collector may include a conductive carbon coating; and/or

the method may further include the step of heating the current collectorat a selected temperature for a selected time prior to the laminatingstep; and/or

the selected temperature may be between about 120° C. and about 300° C.;and/or

the selected temperature may be about 280° C.; and/or the selected timemay be between about 5 minutes and about 120 minutes; and/or

the selected time may be about 30 minutes.

According to another aspect of the disclosure, a method for fabricatinga battery electrode includes forming a mixture including an electrodematerial and a binder; forming an electrode blank from the mixture;heating a current collector having a conductive coating at a selectedtemperature for a selected time to form an annealed current collector;and laminating the annealed current collector to the electrode blank;and/or

the laminating step may include laminating a first electrode blank to afirst side of the annealed current collector and laminating a secondelectrode blank to a second side of the annealed current collector;and/or

the selected temperature may be between about 120° C. and about 300° C.;and/or

the selected temperature may be about 280° C.; and/or

the selected time may be between about 5 minutes and about 120 minutes;and/or

the selected time may be about 30 minutes.

According to a further aspect of the disclosure, a battery electrode maybe made by any of the methods described above.

According to a still further aspect of the disclosure, a batteryincludes a housing; and a cell stack disposed in the housing, the cellstack including an anode, a cathode made by any of the methods describedabove, and a separator electrically insulating the anode from thecathode.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. (canceled)
 2. A method for fabricating a battery electrode, themethod comprising: forming a mixture including an electrode activematerial and a binder; forming an electrode blank from the mixture;heating a current collector having a conductive coating at a selectedtemperature for a selected time to form an annealed current collector;and laminating the annealed current collector to the electrode blank. 3.The method as claimed in claim 2, wherein the conductive coatingincludes a conductive material and a thermoplastic binder.
 4. The methodas claimed in claim 3, wherein the thermoplastic binder includes a wax.5. The method as claimed in claim 3, wherein the conductive material iscarbon.
 6. The method as claimed in claim 3, wherein the selectedtemperature is sufficient to reflow the conductive coating.
 7. Themethod as claimed in claim 2, wherein the selected temperature isbetween about 120° C. and about 300° C.
 8. The method as claimed inclaim 7, wherein the selected temperature is about 280° C.
 9. The methodas claimed in claim 7, wherein the selected time is between is betweenabout 5 minutes and about 120 minutes.
 10. The method as claimed inclaim 9, wherein the selected time is about 30 minutes.
 11. The methodas claimed in claim 2, wherein the selected temperature is about 280° C.and the selected time is about 30 minutes.
 12. The method as claimed inclaim 2, wherein the laminating step includes laminating the annealedcurrent collector between two of the electrode blanks.
 13. The method asclaimed in claim 2, wherein the laminating step is conducted at roomtemperature.
 14. A battery, comprising: a case; and a cell stackdisposed in the case, the cell stack including: an anode; a cathodeincluding a current collector having a conductive coating including aconductive material and a thermoplastic binder, the current collectorhaving been annealed at a predetermined temperature for a predeterminedtime, and an electrode blank laminated to the current collector, theelectrode blank including an electrode active material and a binder; anda separator electrically insulating the anode from the cathode.
 15. Thebattery as claimed in claim 14, wherein the conductive material iscarbon.
 16. The battery as claimed in claim 14, wherein thethermoplastic binder is a wax.
 17. The battery as claimed in claim 14,wherein the cathode includes the current collector laminated between twoelectrode blanks.
 18. The battery as claimed in claim 14, wherein theelectrode blank has been annealed at a selected temperature for aselected time prior to being laminated to the current collector.
 19. Thebattery as claimed in claim 14, wherein the current collector has beenannealed to a temperature sufficient to reflow the conductive coating.